For incandescent lamps, energy is expended in bringing the filament up to its operating temperature The energy supplied to the filament is converted into visible light (short wave radiation) and heat (long wave radiation). The majority of the energy is emitted as heat. Filamented lamps then are necessarily inefficient producers of visible light.
Typical metal halide lamps for commercial lighting produce from 80 to 100 lumens per watt. Even at powers as low as 30 watts, the efficiency can be greater than 50 lumens per watt.
The advantages of an efficient source extend beyond reduced energy consumption. With less heat being generated low cost materials, and assemblies may be used to form the reflectors and lens for the lamps. Smaller, more compact devices are more easily integrated into difficult design conditions, such as those for an aerodynamic lamp.
FIG. 1 shows a cross section of a conventional low pressure electrodeless lamp 10 as known in the prior art. A coupling probe 12 in the form of a rod having an input end and a radiant end is positioned axially in a cylindrical outer conductor 14. Intermediate, and possibly at the axial ends of the center and outer conductors is a volume of gas 16 contained in a transmissive envelope 18, such as a glass envelope. When a radio frequency signal is applied between the coupling probe and the outer conductor, the enclosed gas 16 can be caused to radiate visible, or ultraviolet light. In the later case, the glass envelope may be phosphor coated to generate visible light in the same fashion as a fluorescent lamp. Presently, electrodeless lamps in demonstration are capable of efficiencies of about 110 lumens per watt for an input of about 10 to 20 watts of radio frequency power.
One application of an electrodeless lamp is as a replacement for a standard incandescent lamp. To fabricate a compatible replacement lamp, a suitable power source is needed to convert the alternating current line power to microwave power. If a solid-state power source is used, a serious potential problem may occur in interfacing the lamp with the power source. Since present low-pressure electrodeless lamps require approximately 10 watts of incident power to start, the solid state power source must be capable of delivering a power wave of 10 watts to the off state impedance of the lamp. This power requirement is true even if the starting power wave is the result of multiple reflections. There is then a need for an electrodeless lamp with a low impedance for starting.
Since the off state impedance of existing low-pressure electrodeless lamps is approximately a pure reactance, the power source is required to deliver power to the lamp load with an infinite voltage standing wave ratio (VSWR) unless a buffer, such as an attenuator or ferrite isolator is inserted between the power source and the load. Once the lamp starts, the gas is excited and conducts, with the result that the lamp impedance drops dramatically. The demand for an infinite VSWR for starting followed by a normal or low impedance means the power source must be able to withstand the large impedance mismatch, while providing high output power. The conventional electrodeless lamp design then results conflicting needs for high power and large impedance mismatch, which is conventionally resolved by designing an expensive power supply. There is then a need to for an electrodeless lamp that has low starting impedance, and good impedance matching at full power.
Examples of the prior art are shown in U.S. Pat. Nos. 3,763,392; 3,942,058; 3,943,401; 4,001,631; 4,001,632; 4,002,943; 4,002,944; 4,041,352; 4,063,132; 4,266,166; 4,266,167; 4,427,921; 4,189,661; 4,427,923; and 4,812,702.
U.S. Pat. No. 3,763,392 Hollister broadly shows a light transmissive sphere containing a high pressure gas that is induced to radiate by an induction coil surrounding the sphere.
U.S. Pat. No. 3,942,058 Haugsjaa shows an electrodeless lamp with means for controlling the discharge to prevent attachment to the envelope.
U.S. Pat. No. 3,943,401 Haugsjaa shows an electrodeless lamp with means for moving a conductor to thereby alter the impedance of the lamp between starting and operating states.
U.S. Pat. No. 4,001,631 McNeill shows an electrodeless lamp with means for adjusting the axial penetration of the inner conductor to thereby alter the impedance of the lamp between starting and operating states.
U.S. Pat. No. 4,001,632 Haugsjaa shows an electrodeless lamp with a network means for impedance matching.
U.S. Pat. No. 4,266,166 Proud shows an electrodeless compact fluorescent lamp with a pear shaped envelope described as being a re-entrant cavity. The cavity is considered a re-entrant gas cavity, but not an electrically resonant cavity. This particular patent is considered a good reference of the technology in general and the discussion therein is hereby included by reference.
U.S. Pat. No. 4,427,921 Proud shows an electrodeless ultraviolet light source with a pear shaped cavity denoted as a re-entrant cavity. The cavity is considered a re-entrant gas cavity, but not an electrically resonant cavity.
U.S. Pat. No. 4,189,661 Haugsjaa for Electrodeless Fluorescent Light Source shows a lamp with an outer conductor that is tubular, with and inner rod shaped probe. The gas containment cavity is shown as a cup like structure. The cavity is described as being re-entrant. The cavity is considered a re-entrant gas cavity, but not an electrically resonant cavity.
U.S. Pat. No. 4,002,943 Regan shows an electrodeless lamp with an adjustable microwave cavity. The cavity is designed to be expandable or contractible by threading two wall portions together.
U.S. Pat. No. 4,002,944 McNeill discloses an electrodeless lamp using a resonant cavity to contain the lamp capsule. A tuning element is inserted in the cavity to adjust the cavity resonance.
U.S. Pat. No. 4,041,352 McNeill shows an electrodeless lamp with an included capacitor to assist in lamp starting. On ignition, a switch disconnects the capacitor, allowing full power to flow to the discharge gas.
U.S. Pat. No. 4,063,132 Proud shows an electrodeless lamp with a means for rapidly, and repetitively charging the lamp electrodes.
U.S. Pat. No. 4,427,923 Proud shows an internal electrodeless lamp for radiating ultraviolet light in combination with an exterior envelope with a fluorescent coating material to transform the ultraviolet light to visible light.
U.S. Pat. No. 4,812,702 Anderson discloses a toroidal coil for inducing a toroidal discharge in a containment vessel. Anderson emphasizes the use of a V shaped torus cross section.